Controllable hysteresis damping

ABSTRACT

The invention relates generally to spacecraft attitude stabilization in which magnetic hysteresis damping rods are employed to damp rapid or relatively large oscillations of a spacecraft. In particular, the invention provides a method and means for inducing a local magnetic field within the damping rods after substantial damping of the large oscillations in order to reduce the effect of the earth&#39;&#39;s magnetic field on the rods, thus eliminating persistent low-order oscillations of the spacecraft due to continuing dipole interaction of the rods with the magnetic field of the earth. According to one embodiment of the invention, electrically conductive coils disposed around a magnetically permeable hysteresis damping rod are electrically energized, thus inducing within the rod a local magnetic field of sufficient magnitude such that the earth&#39;&#39;s field produces only a slight and substantially invariant magnetic induction in the rods.

United States Patent Wyatt [4 1 Oct. 17,1972

[ 1 CONTROLLABLE HYSTERESIS DAMPING [72] Inventor: Theodore Wyatt, UnionBridge, Md.

[73] Assignee: The United States of America as represented by theSecretary of the Navy- [22] Filed: Oct. 7, 1970 [2]] Appl. No.: 78,772

[52] US. Cl. ..244/l AS, 336/155 [51] Int. Cl ..B64c HOlf 21/08 [58]Field of Search ..244/1 SA, 1 SS, 1 R; 310/93;

[56] References Cited UNITED STATES PATENTS 3,497,160 2/1970 Mobley..3l0/93 2,953,739 9/1960 Duinker ..336/l55 3,114,518 12/1963 Fischell..335/209 Primary Examiner-Richard E. Aegerter Attorney-Richard S.Sciascia and J. A. Cooke [57] ABSTRACT The invention relates generallyto spacecraft attitude stabilization in which magnetic hysteresisdamping rods are employed to damp rapid or relatively large oscillationsof a spacecraft. In particular, the invention provides a method andmeans for inducing a local magnetic field within the damping rods aftersubstantial damping of the large oscillations in order to reduce theeffect of the earths magnetic field on the rods, thus eliminatingpersistent low-order oscillations of the spacecraft due to continuingdipole interaction of the rods with the magnetic field of the earth.According to one embodiment of the invention, electrically conductivecoils disposed around a magnetically permeable hysteresis damping rodare electrically energized, thus inducing within the rod a localmagnetic field of sufficient magnitude such that the earth's fieldproduces only a slight and substantially invariant magnetic induction inthe rods.

15 Claims, 10 Drawing Figures PATENTEDum 11 1972 SHEET 1 OF 4 INVENTOR.THEQORE WYATT .O. l ORNEY H+ FIG 4 PATENTEDUCT I 7 1912 SHEET 2 OF 4INVENTOR.

THEODORE WYATT PATENTEUUBT 1 1 m2 SHEET 3 OF 4 CONTROLLABLE HYSTERESISDAMPING BACKGROUND AND SUMMARY OF THE INVENTION Control of satellitemotion is accomplished by reaction to an exerted force. In theenvironment of a satellite, some entity must be chosen against whichthat force may be exerted. Although several media, such as gasmolecules, photons, or the earths gravitational field, are available, tobe acted against, in the environment of an earth satellite therelatively large force that can be exerted on the satellite through useof the earth s magnetic field causes this media to be one of the mostfeasible means for motional control.

Attitude stabilization of the final stage of rocket propulsion duringorbital injection is frequently accomplished by spinning the rocketstage and attached satellite about the thrust axis during launch. Sincethe frequency of radio transmission from an orbiting satellite ismodulated by the spin rate of the satellite, this motion must be removedfrom orbiting vehicles which require a high order of frequencystability. In order to remove these disruptive satellite motions,various damping systems have been provided. The magnetic hysteresis lossof ferromagnetic materials has been found to be a useful process fordamping the angular motions of earth satellites. The principle ofhysteresis damping is the conversion of kinetic energy into thermalenergy which occurs when the magnetizing force applied to aferromagnetic material is alternately increased and reduced by themotion which is to be damped. The magnetizing force employed is thenatural field of the earth. The damping provisions in the satellitesconsist of orthogonally-oriented rods of an iron alloy selected for verylossy magnetic hysteresis properties. Presently magnetic hysteresis lossdamping is employed to damp the spin stabilization or tip-off motions ofsatellites resulting from launching into orbit, and the oscillations ofsatellites stabilized in attitude by magnetic or gravity gradienttorques.

The invention provides a method and means for utilizing the beneficialcharacteristics of magnetic hysteresis loss in damping rapid or largeamplitude motions, while at the same time reducing the undesirably largeresidual oscillations in the final stage of damping. The inventionrelates to selective alteration of the character of magnetic hysteresisdamping rods, particularly for gravity gradient stabilized satellites.One embodiment of the present method involves applying to each rod anelectrically induced magnetizing force sufficiently larger than themaximum component of the earths field experienced in orbit so that thevariations in the earths total field intensity and dip angle producenegligibly small variations in the induced magnetism, and hence dipolemoment, of the rods. Obviously, it is necessary that the artificiallyinduced magnetism in the rods be produced in a way so that no net dipoleis created which would produce a disturbing torque on the satellite.

Accordingly, it is the primary object of the invention to provide amethod and means for reducing or eliminating the undesirably largeresidual oscillations usually present in the final phases of hysteresisdampmg.

It is also an object of the invention to provide controllable hysteresisdamping of a satellite by selectively altering the hysteresis capabilityof hysteresis material on board the satellite.

It is another object of the invention to provide a large local magneticfield in the damping system so that variations in the magnetizing forceof the earths field (AI-I) produce small changes in the magneticinduction (AB) acting in the system.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating thehysteresis loop of a material in which a magnetizing force, H, isincreased from zero to saturation, subsequently reduced to zero,increased thereafter to saturation with opposite polarity, reduced againto zero, and finally restored to saturation with the initial polarity;

FIG. 2 is a diagram illustrating the cyclic variations in magneticinduction, B, produced in a magnetic hysteresis rod in a spinningsatellite, wherein the rods have their long axes normal to the spin axisof the satellite and wherein the rods alternately point in approximatelynorth and south directions;

FIG. 3 is a diagram illustrating the continuing interaction between therods and the earths magnetic field after damping of large oscillatorymotion of the satellite;

FIG. 4 is a diagram illustrating the combined effect of a local magneticfield plus the same variation in the earth s magnetic field as shown inFIG. 3;

FIG. 5 is a schematic of a direct current winding on a hysteresis rodfor controlling the incremental permeability of the rod;

FIG. 6 is a schematic of an alternating current winding on a hysteresisrod for controlling the incremental permeability of the rod;

FIG. 7 is a schematicof a reversing direct current through a winding ona hysteresis rod for controlling the incremental permeability of therod;

FIG. 8 is a schematic of a rod arrangement in a satellite for eachdamped axis of motion; and,

FIG. 9 is a schematic of a controllable rod arrangement wherein cores atthe ends of the rods are magnetizable through a capacitive circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The effect on a rod rotating inthe earths magnetic field has been previously described in the art. Asthe rod rotates, it experiences a magnetic field along its length whichvaries sinusoidally at the rate of spin and with a peak amplitude equalto the intensity of the earths magnetic field normal to the axis ofspin. As the magnetic rod undergoes changes in the magnetic field, ittraverses its hysteresis loop. FIG. 1 diagrammatically illustrates theproperty of magnetic hysteresis. The diagram is a plot of themagnetizing force, H, to which a rod or other magnetically permeablematerial is subjected and the resulting magnetic induction, B, producedwithin a rod.

If a test were initiated with zero magnetizing force and an initiallyunmagnetized rod, the condition would correspond to point A on FIG. 1.As the magnetizing force is gradually increased with a polarityarbitrarily designated the magnetic induction increases, slowly at firstand then more rapidly to point B. Further increases in magnetizing forceresult in constantly decreasing gains in magnetic induction. Finally acondition known as saturation induction, point C, is attained whereinfurther increases in magnetizing force result in no further increase inmagnetic induction.

If the magnetizing force is then gradually reduced, the induction alsodecreases, but proportionally less than the force decreases. As themagnetizing force is continuously decreased to zero and then increasedin the direction, the induction reverses polarity. With sufficientincrease of the induction toward the saturation induction is againattained (point D). Change of the magnetizing force back in the sensegradually reduces and eventually reverses magnetic induction. After thepoint of intersection with the original magnetization cruve is reached(point B) the initial curve is retracted. Repetition of the entireprocess repeatedly follows the path of the solid line.

It is significant that, as the magnetizing force, l-l, approaches thesaturation value, the incremental permeability AB/AH becomes vanishinglysmall. The present invention utilizes this natural phenomenon byproviding a large local magnetic field so that variations in themagnetizing force of the earths field (AI-I) produce small changes inthe magnetic induction (AB).

Ordinarily damping rods are constructed of a ferromagnetic materialcapable of magnetic induction sufficiently high that the comparativelyweak field of the earth does not produce saturation. Consequently, if asatellite is spinning or tumbling in orbit, with with a rate of rotationrapid compared to the orbital motion, magnetic hysteresis rods disposedwith their long axes normal to the spin axis and alternately pointing inapproximately north and south directions will undergo cyclic variationsin magnetic induction of the nature shown by a solid line in FIG. 2, thesaturation curve being shown in phantom. The area enclosed by the solidline is a measure of the amount of kinetic energy converted to thermalenergy in each revolution. Since the rotational kinetic energy isreduced by each revolution, the rotation rate is gradually reduced,finally becoming relatively small.

In the usual case the spinning or tumbling is gradually damped to thepoint that the angular motion becomes an oscillation of less than 180peak-to-peak, opposed by the restoring torque of an attitudestabilization process, such as magnetic or gravity gradient attitudecontrol. Because of the smaller amplitude of angular motion the rodscannot reverse magnetic orientation end-for-end in a usual satelliteconfiguration, except perhaps in the vicinity of the earths magneticpoles where the field lines are nearly vertical. Consequently, as thesatellite traverses the orbit the magnetizing force experienced by therods may vary through a range of values or of values without changingsign. Then the hysteresis loop will have the general nature of either ofthe two loops shown as solid lines in FIG. 3.

From the foregoing, two observations are evident. The first is that asdamping proceeds to smaller'oscillations the damping process becomesless effective, or more gentle, as evidenced by the gradual reduction inthe area of the hysteresis loop, whereas damping is more vigorous whenmotions are large in amplitude and rapid in rate. Intuitively, it mayseem that this natural progression in damping characteristic isadvantageous.

The second observation is that, as the motions become progressivelysmaller, the average magnetic dipole moment of the ferromagneticmaterial becomes substantially different from the character exhibitedduring rapid motions of large amplitude. If the motion producesend-over-end orientation of a damping rod with respect to the earthsmagnetic field, then polarity reversals of equal magnitude occur asshown in FIG. 2. The magnetic dipole of a hysteresis damping rod isproportional to the magnetic induction of the rod. The trace of magneticinduction versus magnetizing force follows a symmetric pattern centeredabout zero in this case and, consequently, the average magnetic dipoleis zero. As long as the period of complete induction reversals is briefcompared to the natural period of angular motion of the satellite, theinstantaneous magnetic moment of the rod has no effect on the attitudeof the satellite and provides only the desired damping. However, asdamping progresses and the trace of induction assumes the character ofFIG. 3, the dipole moment of the rod becomes an average value other thanzero and with a polarity causing attraction to the local field of theearth, that is, each rod attempts to align itself in magnetic attitudeorientation. If the plane of the orbit of the satellite is inclined withrespect to the magnetic equator of the earth (a practical necessitysince the plane of the magnetic equator is not fixed in inertial space),the satellite in its orbit passes through varying magnetic dip angles.The magnitude of the dip angle variations is a function of the orbitalinclination, being maximum for a polar orbit.

The combination of a net magnetic moment in a satellite and a varyingmagnetic dip angle around the orbital path produces a changing torquewhich perturbs the attitude of the satellite. This continual attitudedisturbance is particularly noticeable in satellites which arestabilized by the so-called gravity gradient effect. The gravitygradient stabilizing torque is comparatively weak in practical satelliteconfigurations and is particularly ill-conditioned to the control ofsmall attitude variations since the restoring torque varies as the sineof twice the angle off vertical. Hysteresis damping rods, which arenormally deployed orthogonally to the gravity gradient stabilizing boomand hence directly compete with gravity gradient stabilization, producea varying disturbing torque which has a frequency in the vicinity of thepoles of 1.5 revolutions per orbit and 3.0

revolutions per orbit at the equator, whereas gravity gradientstabilization calls for one revolution per orbit. Therefore, it is notsurprising that all satellites employing gravity gradient stabilizationand hysteresis damping by interaction with the earths field demonstrateeffective stabilization and damping until a libration halfangle of 5 to15 is attained. Thereafter the oscillations continue unabated, varyingin an obscurely complex fashion due to the differing frequencies of theseveral processes involved and the cross-coupling generally present.Errors in attitude stabilization of this magnitude are acceptable forsome satellite missions, usually at the price of broader antennapatterns, reduced radio-frequency signal levels, excessive powerconsumption, etc. Examples of such missions are navigation andcommunications relaying. Other missions, such as optical reconnaissanceof the earth and astronomy, are incompatible with poor attitudestabilization. Some of the practical engineering problems in the designof satellites are made more difficult or more uncertain by poor attitudecontrol. Examples include solar-powered generating systems and thermaldesign. Clearly, there would be advantages in obtaining a better qualityof attitude stabilization without the necessity of turning to gas jetreaction .thrusters, horizon scanners, inertial platforms, etc.

The present invention provides a method and means for changing the rodsafter the initial damping phase so that their average magnetic momentdoes not perturb the satellite attitude during the later phase of smallangle librations. The preferred embodiment of the method is to providecommandable means for imposing a magnetizing force on each rod such thatthe earths field produces only slight and substantially invariantmagnetic induction in the rods. In order that no net dipole is createdwhich would compete with the attitude control provisions, such asgravity gradient stabilization, it is necessary that the artificiallyinduced magnetism in the rods take the form of equal dipoles of opposingpolarities or, alternatively, that any net dipole be reversedperiodically with equal dwell time on opposite polarities and at afrequency high compared to the natural frequencies of the modes ofoscillation of the satellite.

The artificially induced magnetism in the rods limits the earths fieldto causing loops to be traced as indicated in FIG. 4. It is evident thatthe area of the hysteresis loop, and consequently the amount of dampingavailable, is markedly reduced in the process. Two steps of artificialinduction are used to tailor the amount of damping retained to thelibration angle remaining. Having reduced the residual angle, thesecond, larger step of artificially induced magnetism would then becommanded. The second step would leave enough damping to cope withnon-magnetic disturbing torques, such as solar pressure. The two stepoperation is indicated as loops 1 and 2 in FIG. 4.

Several embodiments of means useful to accomplish the present method areshown in FIGS. 5 through 9. FIG. 5 illustrates the most simplearrangement wherein a damping rod composed of a magnetically permeablelossy material has a winding 12 of electrically conductive materialwrapped therearound in sets 12a and 12b forming a symmetric pattern ofleft and right hand windings connected in series. Each set 120 and 12bof the windings 12 should have the same number of turns with the samespacing therebetween. The winding 12 is energized on command by a directcurrent source (not shown) to inductively magnetize the rod 10 withoutproducing a net dipole moment. Thus, the hysteresis damping capabilityof the rod 10 is removed by the induced magnetic field within the rod.In effect, the electrically induced magnetizing force applied to the rod10 is sufficiently larger than the maximum component of the earths fieldso that the induced magnetism in the rod produced by the earths field isnegligibly small.

If alternating current is available in the satellite, an electricallyconductive winding 14 having a single degree of rotation can be used onthe rod 10, as shown in FIG. 6. On AC energization of the winding 14,the polarity is reversed at a frequency sufficiently greater than thenatural frequency of the satellite so that the net dipole momentproduced is essentially negligible.

The arrangement of FIG. 6 has the disadvantages of greater heatdissipation and power consumption than the direct current embodimentshown in FIG. 5. However, the arrangement shown in FIG. 7 avoids thesedisadvantages. Direct current is applied to a continuous winding 16 onthe rod 10, the polarity being periodically reversed with an equal dwelltime. The natural period of angular motion of a typical gravity gradientstabilized satellite is approximately one hour. Thus, if the currentreversals are performed at I least as frequently as every six minutes,the attitude of the satellite is not perturbed.

If the satellite contains two rods 18 and 20 for each damped axis ofmotion, the arrangement shown in FIG. 8 is advantageous. In this casetwo electrical windings 22 and 24 of the same number of turns are inseries, the winding 22 being left-handed and the winding 24 beingright-handed. As long as the magnetic properties of the rods 18 and 20are identical, this arrangement provides a neutral magnetic dipole. Ifthere is doubt regarding the rods remaining identical throughout thehazards of assembly, testing and launching, the DC polarity reversalarrangement described-above and referenced to FIG. 7 can be employed asa protective measure. Since the missions of many satellites require anon-board timing system, the automatic switching function described asapplicable to FIGS. 7 and 8 can be readily accomplished withoutsignificant penalty.

Tests of a representative satellite indicate that 50 milliwatts of powerfor each of four rods is adequate to operate any of the direct currentarrangements described hereinabove. However, some satellites do not havethis margin of power available continuously. In this case a chargeablepermanent magnet can be employed. A possible arrangement is shown inFIG. 9.

Controllable rods 26 and 28 have cores 30 and 32 disposed at andintimately contacting their corresponding inner ends, the cores beingcomposed of a ferromagnetic material having good retentivity. Windings34 and 36 of opposite sense are wound one each about the cores 30 and 32and are connected in a circuit 38 which contains a capacitor 40, thecircuit 38 being completed on command by a three-position switch 42.Prior to activation of the controllable rods 26 and 28, the'capacitor 40is charged and then disconnected from a power supply 44. When activationof the rods 26 and 28 is desired, the capacitor 40 is discharged onselective positioning of the switch 42 through the windings 34 and 36.The temporary induction field produced in the cores 30 and 32permanently magnetizes said cores, the magnetic flux being thenconducted into the rods 26 and 28. Cores of ferromagnetic material whichare magnetizable in the manner described may also be disposed at bothends of each of the rods 26 and 28 to provide a lower value of Bl H overthe length of each of the rods for a given maximum value of H, therebyminimizing the magnetic dipole produced by the earths magnetic field.The windings 34 and 36 preferably have opposite senses in order to avoida net dipole from the combined effect of the two rod and magnetizablecore assemblies. The arrangement shown in FIG. 9 requires a high degreeof similarity in the magnetic properties of each pair of the rods andcores. The arrangement shown in FIG. 10 may also be employed if thesatellite contains two rods for each damped axis of motion. Rods 46 and48 have electrical windings 50 and 52 disposed therearound, the windings50 and 52 having the same number of turns and being in the same sense,i.e., being either left-handed or right-handed. The windings 50 and 52are connected in series to a source of D.C. current. As long as themagnetic properties of the rods 46 and 48 are identical, the arrangementdescribed provides a neutral magnetic dipole. If there is doubtregarding the rods remaining identical throughout the hazards ofassembly, testing and launching, the D.C. polarity reversal arrangementdescribed above and referenced to FIG. 7 can be employed as a protectivemeasure.

The means described hereinabove are representative of apparatus whichmay be used to practice the method of the invention, the purpose of thesaid method and means being to minimize pitch and yaw attitudeperturbations. Since mechanical cross-coupling of the motions aboutthree axes (pitch, yaw, and roll) is normally present, the reduction inattitude perturbation is afforded to the roll axis as well as to thepitch and yaw axes. Since shorted inductive winding damping is commonlyemployed in conjunction with magnetic hysteresis damping, shorting theseveral windings described herein in the period prior to controlling thehysteresis damping capability of the rods makes available fshorted coildamping without additional mechanical complexity. I claim: 1. in agravity gradient stabilized satellite employing a magnetic hysteresisstructure to damp relatively large oscillatory motion of the satellite,apparatus for eliminating low-order oscillations of said satellite dueto residual dipole interaction of the magnetic hysteresis structure withthe magnetic field of the earth, comprising means for applying amagnetizing force to the magnetic hysteresis structure throughout saidstructure, thereby inducing a magnetic field in the structure ofsufficient magnitude to minimize the influence of the magnetic field ofthe earth on the structure. 2. The apparatus of claim 1 wherein saidmeans comprises electrically conductive means disposed around thestructure along the full length thereof, and

electrical means for producing a current in the electrically conductivemeans, thereby producing a magnetic field therein.

3. The apparatus of claim 2, and further comprising means to selectivelycontrol the production of electric current in the electricallyconductive means.

4. The apparatus of claim 2, wherein said electrically conductive meanscomprises sets of electrically conductive windings disposedsymmetrically around the structure, alternate sets of windings havingopposite senses and being connected in series, and each set having thesame number of turns therein and having the same spacing between theturns.

5. The apparatus of claim 4, wherein said electrical means comprises asource of direct current.

6. The apparatus of claim 2 wherein said electrical conductive meanscomprises a winding having a single degree of rotation disposed aroundthe structure.

7. The apparatus of claim 6, wherein said electrical means comprises asource of alternating current having a frequency sufficiently greaterthan the natural frequency of the satellite so that the attitude of thesatellite is not perturbed.

8. The apparatus of claim 6, wherein said electrical means comprises asource of direct current and further comprises means for periodicallyreversing the current within the winding, each period of applied currentin both senses being equal in duration, the frequency of the currentreversals being sufficiently greater than the natural frequency of thesatellite so that the attitude of the satellite is not perturbed.

9. The apparatus of claim 2, wherein said hysteresis damping structurecomprises at least two rods oriented with their longitudinal axesextending in the same direction, each of which is composed of amagnetically permeable hysteresis material; the electrically conductivemeans comprising a winding on each of the rods and extending the fulllength thereof, the windings being connected in series, the sense of thewindings being opposite; and the current means comprising a source ofdirect current.

10. The apparatus of claim 2, wherein said hysteresis damping structurecomprises at least two rods oriented with their longitudinal axesextending in the same direction, each of which is composed of amagnetically permeable hysteresis material; the electrically conductivemeans comprising a winding on each of the rods and extending the fulllength thereof, the windings being connected in series, the sense of thewindings being identical; and the current means comprising a source ofdirect current such that the direction of the current flow through therespective windings is opposite.

11. The apparatus of claim 1, wherein the hysteresis damping structurecomprises at least two rods, each of which is composed of a magneticallypermeable hysteresis material, the means for applying a magnetizingforce to-the structure comprising,

a magnetizable core disposed contiguous to at least one end of each ofthe rods,

electrically conductive means disposed around each of the magnetizablecores, and

current means for producing an electric current in the electricallyconductive means in order to induce a magnetic field in the cores, thuspermanently magnetizing said cores.

12. The apparatus of claim 11, wherein said electrically conductivemeans comprise windings on the cores, the windings for any two coresbeing of opposite sense and being connected in series.

13. The apparatus of claim 12, wherein said current means comprises acircuit having a capacitor,

means for charging the capacitor, and

means for connecting the capacitor to said windings to produce currentin said windings.

14. A method for controlling the incremental permeability of ahysteresis damping structure provided aboard an orbiting satellite todamp relatively large oscillatory motion of the satellite, the methodcomprising the step of inducing a magnetic field throughout thestructure of sufficient magnitude to minimize the influence of themagnetic field of the earth on the structure.

15. A method for controlling the incremental energizing the winding by asource of electric current permeability of a hysteresis rod providedaboard an orto induce a magnetic fi ld in the rod f ffi i n ti s fi g g.osclnatory magnitude to minimize the influence of the maglon e Sa e l 6me o compnsmg netic field of the earth on the rod.

disposing an electrically conductive winding around the rod along thefull length thereof, and

1. In a gravity gradient stabilized satellite employing a magnetic hysteresis structure to damp relatively large oscillatory motion of the satellite, apparatus for eliminating low-order oscillations of said satellite due to residual dipole interaction of the magnetic hysteresis structure with the magnetic field of the earth, comprising means for applying a magnetizing force to the magnetic hysteresis structure throughout said structure, thereby inducing a magnetic field in the structure of sufficient magnitude to minimize the influence of the magnetIc field of the earth on the structure.
 2. The apparatus of claim 1 wherein said means comprises electrically conductive means disposed around the structure along the full length thereof, and electrical means for producing a current in the electrically conductive means, thereby producing a magnetic field therein.
 3. The apparatus of claim 2, and further comprising means to selectively control the production of electric current in the electrically conductive means.
 4. The apparatus of claim 2, wherein said electrically conductive means comprises sets of electrically conductive windings disposed symmetrically around the structure, alternate sets of windings having opposite senses and being connected in series, and each set having the same number of turns therein and having the same spacing between the turns.
 5. The apparatus of claim 4, wherein said electrical means comprises a source of direct current.
 6. The apparatus of claim 2 wherein said electrical conductive means comprises a winding having a single degree of rotation disposed around the structure.
 7. The apparatus of claim 6, wherein said electrical means comprises a source of alternating current having a frequency sufficiently greater than the natural frequency of the satellite so that the attitude of the satellite is not perturbed.
 8. The apparatus of claim 6, wherein said electrical means comprises a source of direct current and further comprises means for periodically reversing the current within the winding, each period of applied current in both senses being equal in duration, the frequency of the current reversals being sufficiently greater than the natural frequency of the satellite so that the attitude of the satellite is not perturbed.
 9. The apparatus of claim 2, wherein said hysteresis damping structure comprises at least two rods oriented with their longitudinal axes extending in the same direction, each of which is composed of a magnetically permeable hysteresis material; the electrically conductive means comprising a winding on each of the rods and extending the full length thereof, the windings being connected in series, the sense of the windings being opposite; and the current means comprising a source of direct current.
 10. The apparatus of claim 2, wherein said hysteresis damping structure comprises at least two rods oriented with their longitudinal axes extending in the same direction, each of which is composed of a magnetically permeable hysteresis material; the electrically conductive means comprising a winding on each of the rods and extending the full length thereof, the windings being connected in series, the sense of the windings being identical; and the current means comprising a source of direct current such that the direction of the current flow through the respective windings is opposite.
 11. The apparatus of claim 1, wherein the hysteresis damping structure comprises at least two rods, each of which is composed of a magnetically permeable hysteresis material, the means for applying a magnetizing force to the structure comprising, a magnetizable core disposed contiguous to at least one end of each of the rods, electrically conductive means disposed around each of the magnetizable cores, and current means for producing an electric current in the electrically conductive means in order to induce a magnetic field in the cores, thus permanently magnetizing said cores.
 12. The apparatus of claim 11, wherein said electrically conductive means comprise windings on the cores, the windings for any two cores being of opposite sense and being connected in series.
 13. The apparatus of claim 12, wherein said current means comprises a circuit having a capacitor, means for charging the capacitor, and means for connecting the capacitor to said windings to produce current in said windings.
 14. A method for controlling the incremental permeability of a hysteresis damping structure provided aboard an orbiting satellite to daMp relatively large oscillatory motion of the satellite, the method comprising the step of inducing a magnetic field throughout the structure of sufficient magnitude to minimize the influence of the magnetic field of the earth on the structure.
 15. A method for controlling the incremental permeability of a hysteresis rod provided aboard an orbiting satellite to damp relatively large oscillatory motion of the satellite, the method comprising, disposing an electrically conductive winding around the rod along the full length thereof, and energizing the winding by a source of electric current to induce a magnetic field in the rod of sufficient magnitude to minimize the influence of the magnetic field of the earth on the rod. 